1. Field of the Invention
The present invention relates to a novel method of olefin polymerization that allows for the production of monomodal stereoblock polyolefins of tunable composition, molecular weight, and polydispersity.
2. Related Art
Living polymerizations that occur with reversible deactivation of the propagating centers during propagation, i.e., the reversible formation of inactive or “dormant” species, have now been documented for anionic, cationic, group transfer, and controlled/living free radical polymerizations (Matyjaszewski, K., Macromol. Symp. 174:51-67 (2001), and references cited therein; Müller, A. H. E., et al., Macromolecules 28:4326-4333 (1995); Müller, A. H. E., et al., Macromolecules 28:7335-7338 (1995); Hawker, C. J., Acc. Chem. Res. 30:373-382 (1997)). Slow exchange between active and dormant species, relative to propagation, can lead to a broadening of molecular weight distributions over those for living polymerizations in which such exchange processes are absent (cf., Mw/Mn (polydispersity index, PDI) >1.1 vs. <1.1, respectively) (Matyjaszewski, K., Macromol. Symp. 174:51-67 (2001), and references cited therein; Müller, A. H. E., et al., Macromolecules 28:4326-4333 (1995); Müller, A. H. E., et al., Macromolecules 28:7335-7338 (1995)). On the other hand, when the equilibrium between active and dormant species lies far to the dormant side, the stability, or “livingness,” of the propagating active centers can be greatly enhanced due to their low concentration at any given time. Indeed, the enormous success and popularity of controlled/living free radical polymerization for making well-defined block copolymers and other polymer architectures rests on this fact (Matyjaszewski, K., Macromol. Symp. 174:51-67 (2001), and references cited therein; Hawker, C. J., Acc. Chem. Res. 30:373-382 (1997)). In recent years, a small number of homogeneous transition metal complexes have been reported that can serve as initiators for the living Ziegler-Natta polymerization of α-olefins, and in a few cases, a high degree of stereocontrol during propagation can be achieved, providing either isotactic or syndiotactic polyolefin microstructures (Coates, G. W., et al., Angew. Chem. Int. Ed. 41:2236-2257. (2002)). To date, however, reversible deactivation during propagation, and its potential consequences, have not been documented for a living Ziegler-Natta polymerization process.
Although conceptually simple, it is far from given that a process will either provide an active, living polymerization system, or that narrow polydispersities will be obtained. Schrock and coworkers (Mehrkhodavandi, P., et al., J. Am. Chem. Soc. 122:7841-7842 (2000)) previously found for their homogeneous Ziegler-Natta system that an excess of neutral dimethyl zirconium precursor relative to the borate cocatalyst produced a dead system, presumably due to an equilibrium that lies far to the right of formation of a cationic methyl-bridged dinuclear complex that is inactive for polymerization. Marks and coworkers (Chen, Y. X., et al., J. Am. Chem. Soc. 118:12451-12452 (1996)) further found that, while producing an active, non-living polymerization system, their cationic methyl-bridged dinuclear complex provided broader polydispersities relative to mononuclear cationic initiators, possibly due to slow dissociation and initiation.